The present invention relates to apparatus and methods for reducing line flicker in a television picture. More specifically, for a picture that is reproduced by 100 Hz interlaced scanning, the apparatus and method detects line flicker by comparing vertically successive lines of two temporally successive fields, and corrects line flicker by adaptively delaying or adaptively accelerating corresponding lines within the same field. The term "line flicker" has sometimes been referred to as "edge flicker" and so on.
By convention, television pictures are transmitted to a receiver and reproduced on a screen in accordance with an interlace principle, in which each picture frame is divided into two fields. A first field contains the even-numbered lines of the frame and the second field contains the odd-numbered lines. The two fields are reproduced in an alternating manner by a fast, successive scan of the respective lines. On the European system the fields are transmitted at a rate of 50 Hertz, which results in the frame being reproduced on the screen at a rate of 25 Hz.
In the reproduction of still and slow-motion pictures with sharp dark/light transition areas, visible line jumps may be observed. In the two fields, the transition line between the white/black (light/dark) areas can be reproduced with a mutual spacing of one line. This phenomenon, caused by the alternating reproduction of the two fields, is known as flicker. Occurring at a rate of 25 Hz, the flicker has unpleasant effect on the eye of an observer.
Several methods have been proposed for reducing flicker in an interlaced television picture. For instance, see an article by Schroder et al. entitled "Scanning Modes for Flicker-Free Colour TV Reproduction" in IEEE Transactions on Consumer Electronics, Vol. CE-31, No. 3, pp 627-641 (1985). Three methods are proposed: (1) Motion Adaptive Frame Repeition; (2) Motion Adaptive Progressive Scan Reproduction with Temporal Field Interpolation; and (3) Motion Adaptive Progressive Scan Reproduction with Field Insertion. These three methods reduce line flicker by repeating the fields of the interlaced signal at a 100 Hz rate, which is twice the normal frequency. In 100 Hz interlaced scanning, the temporal field sequence of each frame consists of a primary first field A, secondary first field A', secondary second field B', primary second field B. In method (1), still and slowly moving pictures are reproduced by repeating the fields of a frame in a sequence AB AB. For faster moving areas each field is repeated directly or, where necessary, in a vertically interpolated version. In method (2), a frame is obtained from the incoming interlaced signal by combining the actual field and the temporal mean value of the adjacent fields. In method (3), a frame is obtained from simple addition of two adjacent fields.
Although the reproduction of the fields at a rate of 100 Hz does, in fact, reduce large-area flicker, it does not eliminate or reduce line flicker, which still occurs at a rate of 25 Hz. In the HDTV receiver, the 100 Hz interlaced scanning results in line flicker having an amplitude of two lines, which degrades resolution of the television picture.
See European Patent Application 0 305 017 filed on Aug. 8, 1988 and entitled "A television receiver having means for the suppression of line flicker." A detector detects the distribution of intensity of the video signal in an area vertically above and below the picture spot, and provides a correction signal whenever a significant difference in intensity is detected. This correction signal controls a control unit such that in one or both of the half pictures the line under reproduction will be displaced to such an extent that the transition line in both of the half pictures is reproduced at the same vertical location on the screen.
Another method is described in an article by U. Kraus and entitled "Vermeidung des GroBflachenflimmerns in Fernseh-Heimempfangern" in Rundfunktechnische Mitteilungen 25, H.7, pp. 264-69 (1981). A single repetition of two fields is used to reproduce a frame. The first field of an interlaced signal is reproduced on the screen twice in succession at a rate of 100 Hz. Then, the second field is written on the screen twice. By this method, line flicker is increased to 50 Hz, such that the human eye cannot perceive it. However, such a repetition of the two fields of a frame results in a false assignment of the phase of motion. This causes motion to blur, which drastically reduces the resolution of the television picture. While this method works fairly well for the reproduction of still pictures, it is not well-suited for displaying details in moving pictures.
Yet another method is disclosed in an article by C. Hentschel and entitled "Flimmerruduktionsverfahren zur Verbesserung der Fernseh-Bildwiedergabe" in ntz-Archiv 7, pp. 219-229 (1985). In this motion-adaptive method, first and second fields of a frame are temporarily stored in memory. A motion detector is provided to detect motion in one or more subareas of the first field and in corresponding subareas of the second field. From the temporal and/or vertical changes in subareas of the picture, interpolation coefficients are calculated by means of which a weighted superposition of associated lines of the first and second fields is effected. One disadvantage of this method is that vertical resolution of the television picture is reduced due to the weighted mixture of corresponding lines of the first and second fields. The mixing of the two successive fields, results in motion blur of the television picture. Furthermore, apparatus for implementing this method is extremely complicated and expensive. Since the purchase price of a home receiver equipped with such apparatus would be prohibitive, this method is not suitable for use in home receivers.
Therefore, it is an object of the present invention to reduce line flicker caused by 100 Hz interlaced scanning in a simple and inexpensive manner.
It is a further object of the present invention to ensure that vertical resolution of the television picture is not impaired and that large-area flicker is nor produced.